Water from natural sources often contains dissolved minerals, with an appreciable presence of ions such as Ca2+, Mg2+ and, in the case of oilfield formation water, Ba2+, Sr2+, Zn2+, Pb2+ and Fe2+. Under conditions of temperature or pH change, loss of carbon dioxide from solution or admixture with other water containing different mineral content, relatively insoluble species such as carbonates, sulphates and sulphides may deposit from solution as scale. In offshore oilfields such deposition may be particularly acute when sulphate-containing seawater, pumped underground to aid oil recovery, comes into contact with formation water.
Further to this if a formation water contains sulphide ions, as a result of, but not limited to the presence of hydrogen sulphide (H2S) gas in the reservoir, zinc sulphide (ZnS), lead sulphide (PbS) and iron sulphide (FeS, FeS2) scales can form as a result of zinc, lead or iron containing brines mixing together.
Deposited scale impedes oil recovery and may even become severe enough to block an oil well. It is therefore a common procedure to treat oil wells with a scale inhibitor to minimise or prevent scale deposition. A careful balance of properties must be achieved. The scale inhibitor must not only control scale, but must also on the one hand have sufficient solubility in the waters at the temperatures it will meet to enable placement in the well.
Exotic scales such as zinc sulphide, lead sulphide and iron sulphide can be removed using an acid treatment to restore the rate of oil recovery. However new sulphide scales will be deposited in the well in a short space of time resulting in re-treatment. Acid treatment of exotic scale poses a severe risk due to the generation of H2S gas within the well.